Optical distance measuring devices are typical among devices equipped with light spot position detection devices for detecting a position of a light spot on a solid-state image sensor which spot is formed on the solid-state image sensor by an optical system. In a measurement principle of the optical distance measuring devices, for which triangulation ranging well known to public is used, a position of a received-light spot on the solid-state image sensor is detected and a distance to a measuring object is calculated on basis of change in incident angle of a pencil of reflected light from the measuring object upon the solid-state image sensor according to the distance to the measuring object.
Among such optical distance measuring devices is a distance measuring device disclosed in Patent Literature 1. In the distance measuring device, as shown in FIG. 19, an emission lens 1 and a light receiving lens 2 that have optical axes parallel to each other are placed at a distance of a base length S from each other. A near infrared LED (IRED) 3 is placed on the optical axis of the emission lens 1, and a position sensitive detector (PSD) 4 with a length b having an end face at a distance a from an optical axis of the light receiving lens 2 is placed, on the optical axis of the light receiving lens 2, at a distance as long as a focal length fj of the light receiving lens 2 from the light receiving lens 2.
Light energy emitted from the IRED 3 is reflected by a subject 5 that is at a distance L, passed through the light-receiving lens 2, and imaged on the PSD 4. The PSD 4 has two output terminals 1ch and 2ch. As a result of removal of stationary photocurrent components caused by sunlight or the like, a predetermined relational expression holds between output currents i1, i2 from the output terminals 1ch and 2ch and an incident position x of a reflected spot light. Then i2/(i1+i2) is calculated by a signal processing IC (integrated circuit) that drives the IRED 3 and that calculates distances, so that an inverse number of the distance L to the subject 5 can be calculated.
Among optical distance measuring devices in which a solid-state imaging device is used as a light receiving element in place of the PSD is a distance measuring device for camera disclosed in Patent Literature 2. In the distance measuring device for camera, as shown in FIG. 20, light emitted from an infrared LED 6 passes through an emission lens 7 and reaches a subject 8. Reflection of the light passes through a light receiving lens 9 and is made to be incident on a light receiving element 10 consisting of a solid-state imaging device. On basis of a position of an image of the infrared LED 6 on the solid-state imaging device 10, distance measuring is performed in an active method. The position of the image of the infrared LED 6 on the solid-state imaging device 10 is detected by calculation of light quantity distribution of the light spot imaged on the solid-state imaging device 10.
For optical distance measuring devices, there are various demands to have improved performance in that a wide range of distances from short to long to measuring objects can be measured, in that distances can be measured under environments with strong disturbance light in outdoor or the like, and so on, to be reduced in size, and the like.
The distance measuring device disclosed in Patent Literature 1 is composed of the three chips, i.e., the IRED 3, the PSD 4, and the signal processing IC. Because of low resolving power in the detection of light spot position by the PSD 4, a size of the PSD 4 is required to be comparatively large so as to ensure a large distance S between the light emission and the reception. Accordingly, there are many difficulties in size reduction of the device. In terms of manufacturing cost, it is not suitable to integrate the PSD 4, having the large size, with the signal processing IC.
In the distance measuring device for camera that is disclosed in Patent Literature 2, by contrast, the solid-state imaging device is used as the light receiving element 10 and thus the signal processing circuit can be built into the light receiving element 10. Therefore, the device is superior in feasibility of size reduction in that the device can be composed of two chips, i.e., a light emitting element consisting of the infrared LED 6 and the light receiving element 10 consisting of the solid-state image sensor. Furthermore, the device is superior in feasibility of size reduction in that a distance between the emission lens 7 and the light receiving lens 9 can be reduced because the device is superior to the PSD 4 of Patent Literature 1 in resolving power for light spot position on the light receiving element 10.
In order to make it possible to measure a wide range of distances from short to long, it is required to widen a dynamic range of quantity of light received by the light-receiving element. In the distance measuring device disclosed in Patent Literature 1, in this regard, conditions of integration, light emission and the like are controlled so that output data from the PSD 4 are within a convertible range of A/D converting means.
For improvement in resistance to disturbance light in the distance measuring device disclosed in Patent Literature 1, conditions of integration and the like on the output currents i2 from the PSD 4 are controlled so that saturation of the dynamic range of the A/D converting means is prevented and signal components are extracted from stored data. In the distance measuring device for camera that is disclosed in Patent Literature 2, a visible-light cut-off filter is used and signal components are extracted on basis of differences between presence and absence of the filter.
The conventional optical distance measuring devices, however, have such problems as follows.
In the distance measuring device disclosed in Patent Literature 1, initially, the measurement is performed with change in the integration conditions on the output currents i1, i2 from the PSD 4 according to measurement environment. Accordingly, short distances can be measured comparatively quick, whereas it takes much time to measure long distances. Thus unstable periods of distance measuring cause a problem in that it is extremely inconvenient for downstream equipment which takes in the distance measurements to use the device.
The signal processing with use of the PSD 4 causes another problem in that it is difficult to reduce the size of the device as described above.
On condition that the distance measuring device disclosed in Patent Literature 1 is applied to the solid-state imaging device, a poor quantity of light emitted from the IRED 3 and reflected by an object causes a problem in that difficulty in installation of an analog circuit for extracting only the reflected light that is originally emitted from the IRED 3 from the taken-in light which is a combination of the reflected light and disturbance light, invites poor resistance to the disturbance light.
In the distance measuring device for camera that is disclosed in Patent Literature 2, the resistance to disturbance light is improved by use of the solid-state image sensor as the light receiving element 10. There is a problem, however, in that it is difficult to measure distances short to long in the method using reflected light. Accordingly, measurement of long distances is performed with use of a passive method using a standard part and a reference part that are provided in the light-receiving element.